Processing of Ribosomal RNA of HeLa cells in vitro. The formation of ribosomal RNA involvies the step-wise cleavage of a large precursor RNA found in the nucleolus to smaller RNAs found in mature ribosomes. The precursor RNA is complexed with protein to form pre-ribosomal particles; they contain the complete sequence of mature ribosomal RNA plus 50% disposable sequence. Using such materials as substrate for an in vitro system, a nucleolus-bound ribonuclease has been identified which leads to the formation of particles which appear identical by acrylamide gel electrophoresis to various later stages of ribosome maturation. The purification of this enzyme is proposed. Another enzyme which attacks only double stranded RNAs has been discovered and will be purified and characterized. We will determine its function in processing of RNA.